1. Field of the Invention
The present invention relates to a system for controlling a driving force in a hybrid vehicle which is provided as its prime mover with an internal combustion engine such as a gasoline engine or a Diesel engine and an electric motor such as a motor/generator for outputting a torque when energized by an electric power. More particularly, the invention relates to a hybrid vehicle drive control system which is enabled to synthesize and output the output torque of the internal combustion engine and the output torque of the electric motor by a torque synthesizing/distributing mechanism made of a planetary gear mechanism.
2. Related Art
As the type of the hybrid vehicle employing the aforementioned internal combustion engine and electric motor as its prime mover, there is known a series hybrid type using the internal combustion engine only for driving a generator or a parallel hybrid type using the internal combustion engine for both the power generation and the running power. The drive system of the latter parallel hybrid type is exemplified by a construction in which the output torque of the internal combustion engine and the output torque of the electric motor are synthesized by and outputted from a torque synthesizing/distributing mechanism, as disclosed in Japanese Patent Laid-Open No. 193676/1997 (JP-A-9-193676).
The torque synthesizing/distributing mechanism of the disclosed hybrid vehicle drive control system is constructed of one set of single pinion type planetary gear mechanism, in which the rotary shaft of the motor/generator is connected to a sun gear, in which the output shaft (or crankshaft) of the engine is connected to a ring gear through an input clutch and in which a carrier is connected to the input shaft of a transmission. Between the carrier and the sun gear, moreover, there is interposed an integral clutch for rotating the torque synthesizing/distributing mechanism integrally as a whole.
In the drive system of the prior art, therefore, only the motor/generator is connected to the transmission through the torque synthesizing/distributing mechanism by releasing the input clutch and applying the integral clutch so that the vehicle can be run by the motor/generator. With the input clutch being applied but the integral clutch being released, on the other hand, the planetary gear mechanism constructing the torque synthesizing/distributing mechanism performs the differential action so that the vehicle can be run by adding the output torque of the motor/generator to the engine torque while generating the electric power. If both the integral clutch and the input clutch are applied, moreover, the vehicle can be run while generating the electric power.
Thus, the hybrid vehicle equipped with the torque synthesizing/distributing mechanism can be set with various running modes depending upon how to output the torques of the engine and the motor/generator or how to transmit the torques inbetween. When an accelerator pedal is depressed, for example, to demand a high driving force (or acceleration force), the so-called assist mode, in which the output torque of the motor/generator is added to the engine torque, is set to execute the control to increase the output torque of the motor/generator gradually while increasing the engine output. By performing these controls, a necessary and sufficient driving force can be established.
When a demand for increasing the driving force is made in the drive system of the prior art, the so-called "assist mode" is set so that the torque to be inputted to the transmission is augmented by increasing the engine output and the output of the motor/generator. If, in this case, the speed of the motor/generator is evenly raised to bring the entirety of the torque synthesizing/distributing mechanism finally into an integrally rotating state, the torque to appear at the carrier acting as the output member of the torque synthesizing/distributing mechanism, i.e., the input torque of the transmission may fluctuate in magnitude to deteriorate the riding comfort.
Even if an increase in the driving force is demanded by depressing the accelerator pedal, more specifically, the accelerator pedal may not be always kept at the depressed opening but may be changed in its depression stroke according to the running situations or the taste of the driver. In this case, the engine torque may fluctuate to change the summed output torque of the engine torque and the torque of the motor/generator independently of the change in the accelerator opening. This may change the magnitude of the driving force to deteriorate the riding comfort.